Fiber optic attenuators and attenuation systems

ABSTRACT

Controllable fiber optic attenuators and attenuation systems are disclosed for controllably extracting optical energy from a fiber optic, and therefore attenuating the optical signal being transmitted through the fiber optic. In one aspect, material is removed from a portion of the fiber optic, thereby exposing a surface through which optical energy can be extracted. A controllable material is formed over the surface for controllably extracting optical energy according to a changeable stimulus applied thereto, which affects the refractive index thereof. In an improved embodiment, a controllable material is formed over the exposed surface for controlling the amount of optical energy extracted from the fiber optic, and a bulk material is formed over the controllable material, into which the extracted optical energy is radiated. The controllable material has a controllable index of refraction approximately matching the index of refraction of the cladding of the fiber optic, and the bulk material formed over the controllable material has a fixed index of refraction higher than the effective mode index of refraction of the fiber optic. This cladding-driven design varies the effective optical thickness of the controllable material, and therefore effects a controllable attenuation of the optical signal being transmitted in the fiber optic. Attenuation systems including these controllable attenuators, as well as control and sense sub-systems, are also disclosed.

TECHNICAL FIELD

The present invention relates to controllable attenuators andattenuation systems for attenuating optical energy transmitted through afiber optic.

BACKGROUND OF THE INVENTION

There is often a requirement in fiber optic systems for precise controlof optical signal levels entering various system components. This isparticularly true for systems at test and characterization stages ofdeployment. A controllable optical attenuator can be used, for example,to characterize and optimize the optoelectronic response of high-speedphotoreceivers, wherein the detection responsivity is dependent on theaverage optical power incident on the photodiode.

The majority of controllable fiber optic attenuators currentlycommercially available rely on thin-film absorption filters. Thisrequires breaking the fiber and placing the filters in-line.Controllable attenuation is then achieved by mechanical means such asrotating or sliding the filter to change the optical path length withinthe absorptive material. This adversely impacts the response speed ofthe device, the overall mechanical stability, zero attenuation insertionloss and optical back reflection. In general, broken fiber designssuffer numerous disadvantages such as high insertion loss, significantback reflection, and large size. These factors can be minimized,although such corrective measures typically result in added cost and/orsize.

What is required are improved controllable fiber optic attenuators andattenuation systems which keep the optical fiber core intact and whichachieve controllable attenuation via control of radiative loss from thefiber.

SUMMARY OF THE INVENTION

The present invention relates to controllable fiber optic attenuators(e.g., variable optical attenuators "VOAs") and attenuation systems,designed to operate in the conventional telecommunication spectralwindows of 1300 nm and 1550 nm, or any other wavelengths of interest,especially those at which single mode propagation occurs. The devicescan be placed in fiber optic networks or systems by simple fusionsplicing or connectorization to attenuate optical signal levels by adesired amount. Controllable attenuation is achieved, for example, bythermal or electrical control of controllable material layers. Thedevices can be used for controllable attenuation in fiber optic systemsat the test and characterization stage, or for active control duringoperational deployment.

The side-polished fiber ("SPF") devices of the present invention are animprovement over conventional broken fiber approaches because of theirintrinsic fiber continuity.

In a first embodiment of a controllable attenuator of the presentinvention, a fiber is mounted in a block and polished to within a closeproximity (e.g., a few microns) of the core. A controllable bulkmaterial, with an approximately matched refractive index (to theeffective fiber mode index) is applied over the polished surface.Adjusting the index of refraction of the bulk material (e.g., via theelectro- or thermo-optic effect), results in a controllable amount ofoptical energy extracted from the fiber optic, thus achievingcontrollable attenuation.

An attenuation system, including a controllable attenuator, is alsodisclosed in which a control circuit applies a changeable stimulus tothe controllable material, in accordance with a desired level stimulus,and/or a sensed level stimulus received from a sense circuit coupled tothe fiber optic for sensing a level of optical energy being transmittedtherein.

In an improved embodiment of the controllable attenuator of the presentinvention, the fiber is polished through its cladding almost to itscore, and a thin controllable material is placed between the fiber and ahigh-index, bulk overlay material. The index of refraction of thecontrollable material (approximately matched to that of the cladding) isvaried, which effectively varies the effective optical thickness (indexx actual thickness) of the remaining cladding. This improved,cladding-driven ("CD") controllable attenuator provides nearlyspectrally flat optical attenuation in the wavelength ranges ofinterest, while retaining all of the intrinsic advantages of the SPFarchitecture. Moreover, a design is disclosed where the typically usedradius block holding the fiber is eliminated, which allows the device tobe reduced in size so that it is not much larger than the fiber itself.

In that regard, the present invention relates to, in its firstembodiment, an attenuation system for attenuating optical energy beingtransmitted through a fiber optic. A controllable attenuator is arrangedwith respect to a portion of the fiber optic having material removedtherefrom thereby exposing a surface thereof through which at least someof the optical energy can be controllably extracted. The attenuatorincludes a controllable material formed over the surface forcontrollably extracting the optical energy according to a changeablestimulus applied thereto which affects the refractive index thereof. Alevel sensing circuit may be coupled to the fiber optic for sensing alevel of at least a portion of the optical energy transmitted thereinand providing a sensed level stimulus to a control circuit, which iscoupled to the controllable attenuator for applying the changeablestimulus to the controllable material thereof in accordance with thesensed level stimulus received from the level sensing circuit.

The changeable stimulus applied to the controllable material may be, forexample, temperature (thermo-optic effect) or voltage (electro-opticeffect).

In a second, improved aspect, the present invention relates to acladding-driven ("CD") controllable attenuator for attenuating opticalenergy transmitted through a fiber optic. The controllable attenuator isarranged with respect to a portion of the fiber optic having materialremoved therefrom thereby exposing a surface thereof through which atleast some of the optical energy being transmitted therein can beextracted. The controllable attenuator includes a controllable materialformed over the exposed surface for controlling an amount of opticalenergy extracted from the fiber optic according to a changeable stimulusapplied to the controllable material which affects the index ofrefraction thereof. In addition, a bulk material layer formed over thecontrollable material is provided into which the extracted opticalenergy is radiated.

In this embodiment, the controllable material has a controllable indexof refraction approximately matching the index of the cladding, and thebulk material formed over the controllable material has a fixed index ofrefraction higher than the effective mode index of the fiber optic.

The controllable fiber optic attenuators and attenuation systems of thepresent invention are valuable in any applications where control of theoptical power transmission in an optical fiber is required. Theattenuators are especially useful in applications where the spectralflatness of attenuation is a concern. Because of the fiber continuity,these devices exhibit the intrinsic benefits of low insertion loss, lowback reflection (high return loss), polarization insensitivity, smallsize, low cost, and mass produceability.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the following detaileddescription of the preferred embodiment(s) and the accompanying drawingsin which:

FIG. 1a is a side, cross-sectional view of a first embodiment of acontrollable fiber optic attenuator in accordance with the presentinvention;

FIG. 1b is an end cross-sectional view of the controllable attenuator ofFIG. 1a;

FIGS. 2a-b are graphs (in percentage, and decibels, respectively)depicting the loss characterization versus the refractive index of abulk (e.g., liquid) overlay for three exemplary levels of fiberside-polishing;

FIG. 3a is a detailed view of the material interfaces of thecontrollable attenuator of FIGS. 1a-b, and further depicts an exemplarymode profile of the optical energy being transmitted in the fiber optic;

FIG. 3b is a detailed view of the material interfaces of a second,cladding-driven embodiment of a controllable fiber optic attenuator ofthe present invention;

FIGS. 4a-b are respective graphs of the spectral performance of thecontrollable attenuators of FIGS. 3a-b;

FIG. 5 is a graph of resultant attenuation versus the superstraterefractive index of side-polished fiber attenuators, and depicts therespective operating ranges of the controllable attenuators of FIGS.3a-b;

FIG. 6a is a side, cross-sectional view of the second, cladding-drivencontrollable attenuator of FIG. 3b;

FIG. 6b is a side, cross-sectional view of an improvement to thecladding-driven controllable attenuator of the present invention whereinthe cladding is removed from the fiber optic without a radial mountingin a substrate block;

FIG. 7 is a functional block diagram of an exemplary attenuation systemin accordance with the present invention; and

FIG. 8 is an exemplary schematic of the attenuation system of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In accordance with the principles of the present invention, a firstembodiment 100 of a controllable attenuator is depicted in FIGS. 1a-b,in which a single-mode optical fiber 30 (e.g., telecommunicationsCorning SMF-28) is side-polished through its cladding 50 close to itscore 40, thereby exposing, through surface 65, an evanescent tail of theoptical energy transmitted in the fiber. Typically, the remainingcladding thickness is <about 10 μm. Optical energy can be extracted fromthe fiber core by application of a bulk material 60 over the polishedsurface 65 of the fiber cladding. The bulk material should have arefractive index slightly less than or approximately equal to that ofthe fiber's effective mode index n_(ef). This value is dependent uponthe fiber core and cladding indices, and the fiber core dimensions, butusually lies between the core and cladding indices. Maximum opticalenergy is extracted from the fiber when the index of the bulk materialmatches the fiber's effective mode index.

In accordance with the present invention, and as discussed in greaterdetail below, the bulk material may be formed from a material which iscontrollable, e.g., its index of refraction can be varied according to achangeable stimulus applied thereto. In the embodiment of FIG. 1a,temperature or voltage changes can be used, and a controllable heatingelement (or electrodes) 80 is provided, for providing a changeabletemperature (or voltage) stimulus to material 60 in accordance with acontrol stimulus 105.

Discussed below are first, the fabrication of the side-polished fiberportion of attenuator 100 and its subsequent loss characterization;second, alternate embodiments 100' and 100'" of a controllableattenuator; and finally, the implementation of an attenuation systemincluding the controllable attenuator 100 (or 100' or 100"), in additionto other control sub-systems.

Side-Polished Fiber Fabrication/Characterization

Standard single-mode fibers have an 8.3 μm diameter core region 40 ofslightly raised refractive index surrounded by a 125±1 μm fused silicacladding 50. The mode field diameter is 9.3±0.5 μm at 1310 nm and10.5±0.5 μm at 1550 nm. The refractive index values supplied by Corningfor SMF-28 fiber are:

    λ=1300 nm: n.sub.core =1.4541, n.sub.clad =1.4483

    λ=1550 nm: n.sub.core =1.4505, n.sub.clad =1.4447

The small difference between the core and cladding refractive indicescombined with the small core size results in single-mode propagation ofoptical energy with wavelengths above 1190 nm. Therefore, the fiber canbe used in both spectral regions although it was designed for 1310 nmoperation where dispersion (combination of material and waveguidedispersion) is minimized and attenuation is low (<0.4 dB/km).

The side-polished fiber controllable attenuator of FIGS. 1a-b may befabricated by lapping and polishing techniques. The fiber is embedded ina fused silica substrate block 20 containing a controlled radius groove.Material is carefully removed from the fiber cladding 50 until the core40 is approached. At this point, the evanescent field of the opticalenergy transmitted in the optical fiber can be accessed through surface65. The device interaction length can be controlled by the remainingcladding thickness and the groove's radius of curvature.

Once the fiber core has been approached via the lapping/polishingprocess, a multiple liquid-drop procedure may be performed tocharacterize the loss of the side-polished fiber. This procedureinvolves placing a series of bulk overlays (e.g., liquids, oils) ofknown refractive index onto the polished surface of the fiber. This hasthe advantage that the interface between the oil and the side-polishedfiber is always as good as the fiber surface and there is no need totreat the surface/oil interface in any special way.

A set of Cargille Refractive Index Liquids is available withwell-characterized refractive indices and dispersion curves. Thus, anaccurate loss/refractive index characterization of each fabricatedside-polished fiber can be obtained. Each liquid used in themeasurements has a specified n_(D) value, where subscript D denotes theSodium D-Line wavelength (λ=589 nm). Dispersion equations are availablewhich allow the response to be adjusted to the spectral region ofinterest i.e. 1300 nm or 1550 nm. FIGS. 2a-b show the optical energytransmission in percentage and decibels, respectively, versus theliquid's refractive index response for three side-polished fibers whicheach have different remaining cladding thicknesses (i.e., 24%, 65% and91% polished cladding levels). At liquid indices below the fiber modeeffective index (n_(ef)), no optical power is removed from the fiber.Close to n_(ef), the transmission response drops sharply and strongextraction is observed. Above n_(ef), the fiber transmission graduallyapproaches a set level of attenuation.

Prior to any cladding removal, the fiber guides light efficiently. Whenpart of the cladding is removed, a new cladding exists which is composedof a small thickness of fused silica surrounded by air (n=1). Since thiscomposite cladding has an effective cladding index less than that of thecore, the fiber can still operate efficiently as a waveguide. This istrue for those overlays having indices less than the fiber modeeffective index, and 100% optical energy transmission therefore occurs.However, when the liquid index is raised above n_(ef), the fiberoperates as a leaky waveguide and a bulk wave is excited in the liquid.Thus, power is leaked from the fiber within the interaction region and acertain attenuation occurs. The efficiency of coupling to the bulk waveis maximum when the liquid's index matches the fiber mode effectiveindex n_(ef). This efficiency is reduced when the liquid's index isincreased above n_(ef), although a significant fraction of power isstill coupled out of the fiber.

Transmission measurements can be made using Fabry-Perot Diode Lasers at1300 nm and 1550 nm and a well-calibrated Optical Power Meter. Strongerattenuation figures are observed for the same liquid index at 1550 nmsince the evanescent penetration of the fiber mode field into thecladding is greater at the longer wavelength.

In accordance with the present invention, as discussed above, a bulkmaterial 60 is applied over the exposed surface of the side-polishedfiber optic. The bulk material 60 is, for example, a controllablepolymer (e.g., electro-optic or thermo-optic) with an index ofrefraction closely matched to the effective mode index of the fiber, andwhich exhibits a change in refractive index proportional to a change in,e.g., temperature or voltage. OPTI-CLAD® 145 available from OpticalPolymer Research, Inc. is an example of such a polymer. A controllableattenuator (100, FIGS. 1a-b) is therefore formed capable of extracting acontrollable amount of optical energy from the fiber. Control of theattenuation is provided by heating element (or electrodes) 80 controlledusing a control stimulus 105.

To achieve the maximum thermo-optic responsivity, for example, thecontrollable attenuator is implemented to exploit the most sensitivecharacteristic refractive index response of the side-polished fiber,determined as set forth above. This occurs when the refractive index ofthe bulk material is slightly less than the effective mode index of theoptical fiber (e.g., n_(ef) =1.449), i.e., proximate the vertical lines99 drawn on the graphs of FIGS. 2a-b. These lines 99 therefore describein general the theoretical operating range of this first embodiment of aside-polished fiber controllable attenuator.

Alternate Controllable Attenuator Embodiments

One aspect of the above-discussed controllable attenuator embodiment 100is that the level of attenuation may vary with wavelength, which maycause design problems for multi-wavelength transmission systems.

In accordance with the present invention, an improved, cladding-driven("CD") side-polished fiber controllable attenuator is disclosed whichimproves spectral performance while retaining all of the intrinsicperformance strengths of non-invasive, side-polished fiber devices.

FIGS. 3a-b respectively depict in detail the material interfaces of thebulk overlay controllable attenuator 100 discussed above, and theimproved, cladding-driven controllable attenuator 100' of the presentinvention. With reference to FIG. 3a, controllable attenuator 100includes a fiber core 40, a remaining portion of cladding 50 (thicknessC_(th), e.g., <about 10 μm) having an exposed surface 65 thereof throughwhich optical energy is extracted into controllable bulk material 60. Amode profile 90 is also depicted approximating the amount of opticalenergy present in the material layers, including the evanescent tail 91(the penetration of which into layer 60 is controllable as set forthabove).

The cladding-driven controllable attenuator 100' of FIG. 3b alsoincludes a fiber core 40', but remaining portion 50' of the cladding(thickness C_(th) ', e.g., <about 2 μm) is a very thin layer and a thinfilm(e.g., thickness <about 10 μm) of controllable material 60' ispositioned over cladding 50'. A bulk material 70' is positioned overlayer 60' and is a high index material. Evanescent tail 91' of modeprofile 90' penetrates through exposed surface 65' into high index layer70' at a depth determined by the effective optical thickness (index xactual thickness) of controllable material 60', which has an indexapproximately matched to that of the cladding. This effective opticalthickness of layer 60' (index x actual thickness) is controlled byvarying the refractive index thereof according to the techniquesdiscussed above, e.g., thermo-optic or electro-optic effects.

The most significant differences between the cladding-driven embodiment100' and the controllable bulk material embodiment 100 are: (i) most ofthe fiber cladding is initially removed (on the polished side) andreplaced with a cladding index matched, but controllable thin layer ofmaterial 60' (e.g. a thermo-optic polymer having an index of about 1.447at 1300 nm) and (ii) the bulk overlay 70' is of higher index, forexample, silicon, with an index of about 3.5.

As shown in the graphs of FIGS. 4a-b, which respectively represent thespectral performance of controllable attenuator embodiments 100 and100', these improvements result in a better spectral uniformity. Thereasons for this spectral uniformity can be understood by referring tothe respective operating ranges 99 and 99' of the attenuation graph ofFIG. 5. The attenuation of a side-polished fiber device is a sensitivefunction of both: (i) the remaining cladding thickness; and (ii) theindex of any overlay material. In the first embodiment of controllableattenuator 100, a significant portion of the evanescent tail of thefiber mode profile propagates within the remaining cladding. Thereforeto achieve significant attenuation, the side-polished fiber is overlaidwith a bulk material 60 which has a refractive index which lies close tothe effective fiber mode index, n_(ef). Adjusting the bulk materialindex produces an attenuation transfer function which follows the verysharp edge of the attenuation response curve, i.e., proximate verticalline 99.

However, because this edge is so sharp, the amount of attenuation isvery sensitive to variations in the fiber mode profile. Therefore,effects such as dispersion (changes in refractive index versuswavelength), can result in wavelength dependent performance. Another,perhaps more significant effect occurs simply because the fiber modeitself is larger at long wavelengths. This results in increasedevanescent penetration into the overlay, and thus higher attenuation.

The cladding-driven controllable attenuator embodiment 100' eliminatesthese effects because its operation is based on an entirely differenttransfer function. As shown toward the right side of FIG. 5, thecladding-driven approach adjusts the effective optical thickness of theremaining cladding using the controllable, cladding index-matched layer60', and therefore changes the amount of evanescent tail 91' penetrationinto bulk material 70', which has a fixed, high index. The attenuationis therefore much less sensitive to variations in the refractive indexof the bulk material when that index lies far above n_(ef). Thecladding-driven device therefore operates along vertical line 99' towardthe right side of FIG. 5. This has been shown to produce attenuationlevels which are nearly independent of wavelength (FIG. 4b), andtherefore improves the spectral uniformity of the device.

The index insensitivity of the bulk material 70' also implies that for agiven amount of remaining fiber cladding (which determines the amount ofattenuation at high index for a given interaction length), varying theindex of the bulk material 70' (e.g through the thermo-optic effect)will not significantly alter the amount of attenuation. Therefore theresponse of such a device without a controllable cladding layer would benegligible.

The solution to this apparent impasse was found by observing that theamount of attenuation (with a high index bulk material) is verysensitive to the amount of remaining thickness of fiber cladding; i.e.,the more cladding removed, the higher the resulting attenuation (asshown by the curves toward the right side of FIG. 5). Thus, if anSPF-based device is produced which operates along the transfer function99' toward the right side of FIG. 5, then both high device responsivityand spectral flatness can be realized. The cladding-driven controllableattenuator 100' achieves these results.

In the cladding-driven controllable attenuator 100', nearly all of theoriginal (silica) fiber cladding is removed (typically by polishing,although chemical etching is possible). This would normally result ina >99% (>-20 dB) high index overlay coupler. However, the removedcladding is replaced with a thin film of controllable material 60'(similar in thickness to the evanescent penetration depth) which has asimilar (fiber cladding matched) ambient refractive index. Further, therefractive index of this material is much more responsive to an appliedsignal (e.g. thermo-optic: heat, or electro-optic: voltage), than thatof the original silica cladding. On top of this thin layer, a high indexbulk material 70' is applied to preserve spectral flatness, as discussedabove.

Under ambient conditions, a device with very low attenuation results.However, by applying a changeable stimulus to the "replacement" claddinglayer 60' which raises its index (up to that of the effective modeindex), the evanescent mode penetration through this "replacement"cladding layer 60' can be varied, and therefore the depth of itspenetration into the high index bulk overlay 70', effecting controllableattenuation. Removing the stimulus reduces the refractive index of thereplacement cladding layer 60', which restores low loss transmission.Any induced variations in the refractive index of the bulk material 70'are negligible because of the intrinsic insensitivity of the device tothis parameter. Thus, the cladding-driven controllable attenuator 100'simultaneously achieves high responsivity and spectral flatness, as wellas low insertion loss, low back reflection, small size, and low losscharacteristics of SPF-based devices, all of which make this embodimenthighly attractive.

Side, cross-sectional views of two potential embodiments (100' and 100")of cladding-driven controllable attenuators are shown in FIGS. 6a-b. Theembodiment of FIG. 6a, discussed generally above, is a design based onthe typical SPF radius groove block, wherein the radius of the fiber,upon its polishing, results in a flat surface 65' though which opticalenergy can be extracted. FIG. 6b depicts an alternative blockless design100" which is fabricated by removing material to produce a radialsurface 65", over which controllable material 60" and bulk material 70"are conformably formed, up to the outer diameter of the fiber. Cladding50" remains (thickness C_(th) " of <about 2 μm). Elimination of the SPFblock in the design 100" allows the device to be reduced in size, sothat it is not much larger than the fiber itself.

Those skilled in the art will recognize that embodiment 100 discussedabove can also be fabricated using this blockless design.

Attenuation System(s) Employing Controllable Attenuators

An exemplary attenuation system 500 employing controllable attenuator100 (or 100' or 100") is shown in FIG. 7. The attenuation system 500includes a controllable attenuator 100 (or 100' or 100"), a controlcircuit 300, and an optional level-sense circuit 200. Control circuit300 supplies control stimulus 105 to the controllable attenuator 100 tochange the changeable stimulus (temperature or voltage) and thereforethe refractive index of the controllable material thereof. Controlcircuit 300 receives as an optional input a desired level stimulus 305from, for example, a user, and adjusts the control stimulus 105 as afunction thereof. Control circuit 300 may also receive an optionalsensed level stimulus from level sense circuit 200. This sensed levelstimulus can be, for example, a ratio of measured levels of opticalenergy both prior to and following the attenuation thereof by theattenuator 100. By comparing this sensed level stimulus to the desiredlevel stimulus, control circuit 300 can vary the value of controlstimulus 105 until the input desired level stimulus and sensed levelstimulus are matched.

Exemplary attenuation system 500 is depicted in an exemplary schematicform in FIG. 8. The controllable attenuator 100 is preceded and followedby 1% fiber couplers (splitters 210, 230) which tap a small fraction ofthe optical power propagating in the fiber. The decoupled light iscarried to characterized photodetectors (220, 240) and the generatedphotocurrents are analyzed by a ratiometer 250. Comparator circuit 310receives the sensed level stimulus output of the ratiometer and/or adesired level stimulus 305 (from a user) and transmits a signal to thetemperature controller 320. The temperature controller provides thecontrol stimulus 105 to controllable attenuator 100 to change thechangeable stimulus (temperature or voltage) and therefore therefractive index of the controllable material thereof. In this way, theoptical attenuation level (photocurrent ratio) is directly compared to acalibrated attenuation adjustment signal 305 (user or system input)until they are matched. This feedback loop controls the attenuationeffected by the controllable attenuator and therefore ensures accurateperformance.

The present invention also extends to the methods for forming and usingthe disclosed controllable attenuators and attenuation systems, andfurther to methods for attenuation, discussed above.

Those skilled in the art will also recognize that the present inventionextends to i.) fixed set point attenuators wherein, under controlledambient conditions, the controllable material layers are designed with apredetermined refractive index such that a predetermined, fixed level ofattenuation results, thereby negating the need for a changeable stimulusapplied to the controllable material, and ii.) adaptive attenuationwherein a fixed attenuation level is desired, and the changeablestimulus is adaptively applied to the controllable material as afunction of changing ambient conditions which unintentionally affect therefractive index of the controllable material.

While the invention has been particularly shown and described withreference to preferred embodiment(s) thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A controllable attenuator for attenuating opticalenergy transmitted through a fiber optic, the controllable attenuatorarranged with respect to a portion of the fiber optic, the portion ofthe fiber optic having material removed therefrom thereby exposing asurface thereof through which at least some of said optical energytransmitted therein can be extracted, the controllable attenuatorincluding:a controllable material formed over the exposed surface forcontrolling an amount of optical energy extracted from said fiber opticaccording to a changeable stimulus applied to the controllable materialwhich affects the index of refraction thereof; and a bulk material,formed over the controllable material, into which the extracted opticalenergy is radiated.
 2. An attenuation system, comprising:thecontrollable attenuator of claim 1; and a control circuit coupled to thecontrollable attenuator for controlling a value of the changeablestimulus applied to the controllable material.
 3. The attenuation systemof claim 2, further comprising:a level sensing circuit, coupled to saidfiber optic, for sensing the level of at least a portion of the opticalenergy transmitted therein and providing a sensed level stimulus to saidcontrol circuit according to said sensed level;wherein the controlcircuit controls the value of the changeable stimulus applied to thecontrollable material in accordance with the sensed level stimulus. 4.The controllable attenuator of claim 1, wherein the material removedfrom the fiber optic is a portion of cladding encasing a core thereof,and wherein:the controllable material has a controllable index ofrefraction approximately matching the index of refraction of thecladding; and the bulk material formed over the controllable materialhas a fixed index of refraction higher than the effective mode index ofrefraction of the fiber optic.
 5. The controllable attenuator of claim1, wherein the changeable stimulus applied to the controllable materialcomprises temperature or voltage.
 6. An attenuation system forattenuating optical energy transmitted through a fiber optic,comprising:a controllable attenuator, arranged with respect to a portionof the fiber optic, the portion of the fiber optic having materialremoved therefrom thereby exposing a surface thereof through which atleast some of said optical energy can be controllably extracted, theattenuator including a controllable material formed over said surfacefor controllably extracting said optical energy according to achangeable stimulus applied to said controllable material which affectsthe refractive index thereof; a level sensing circuit, coupled to saidfiber optic, for sensing a level of at least a portion of the opticalenergy transmitted therein and providing a sensed level stimulus; and acontrol circuit coupled to the controllable attenuator for controllingan amount of the changeable stimulus applied to the controllablematerial in accordance with the sensed level stimulus, received from thelevel sensing circuit.
 7. The attenuation system of claim 6, wherein thelevel sensing circuit includes:a first sensing circuit for sensing anamount of optical energy transmitted in said fiber optic prior to anyextraction thereof by said controllable attenuator; a second sensingcircuit for sensing an amount of optical energy transmitted in saidfiber optic following any extraction thereof by said controllableattenuator; and a circuit for determining a level of optical energyextracted from said fiber optic according to the sensed amounts ofoptical energy prior to and following the extraction thereof by saidcontrollable attenuator, and providing the sensed level stimulus to thecontrol circuit based on said level of optical energy extracted.
 8. Theattenuation system of claim 7, wherein the control circuit includes acomparing circuit for comparing the sensed level stimulus and thedesired level stimulus applied thereto, and, based on any differencetherebetween, changing the value of the control stimulus provided to thecontrollable attenuator.
 9. The attenuation system of claim 8, whereinthe changeable stimulus applied to the controllable material comprisestemperature or voltage, and wherein the control circuit provides acontrol stimulus to said controllable attenuator to change thetemperature or voltage applied to said controllable material.
 10. Theattenuation system of claim 9, wherein:the controllable attenuatorincludes a bulk material formed over the controllable material intowhich the extracted optical energy is radiated.
 11. The attenuationsystem of claim 10, wherein the material removed from the fiber optic isa portion of cladding encasing a core thereof, and wherein:thecontrollable material of the controllable attenuator has a controllableindex of refraction approximately matching the index of refraction ofthe cladding; and the bulk material formed over the controllablematerial has a fixed index of refraction higher than the effective modeindex of refraction of the fiber optic.
 12. The attenuation system ofclaim 9, wherein:the controllable attenuator includes a controllableheating element, or electrodes, having an input for receiving saidcontrol stimulus from said control circuit, and arranged with respect tothe controllable material to change the temperature, or voltage, andtherefore the refractive index thereof in accordance with said controlstimulus.
 13. The attenuation system of claim 6, wherein the changeablestimulus to be applied to the controllable material comprisestemperature or voltage, and wherein the control circuit provides acontrol stimulus to said controllable attenuator to change thetemperature or voltage applied to said controllable material.
 14. Theattenuation system of claim 13, wherein:the controllable attenuatorincludes a controllable heating element, or electrodes, having an inputfor receiving said control stimulus from said control circuit, andarranged with respect to the controllable material to change thetemperature, or voltage, and therefore the refractive index thereof inaccordance with said control stimulus.
 15. A method for controllablyattenuating optical energy transmitted through a core of a fiber optic,a portion of the fiber optic having a portion of cladding encasing thecore removed therefrom, thereby exposing an evanescent tail of theoptical mode field transmitted through the core, the methodcomprising:extracting optical energy from the evanescent tail of theoptical mode field using a bulk material, positioned over said portionof the fiber optic, into which the evanescent tail penetrates; using acontrollable material positioned between the bulk material and the coreof the fiber optic to vary a depth of penetration of the evanescent tailinto the bulk material, including varying the index of refraction of thecontrollable material; and sensing a level of at least a portion of theoptical energy transmitted in said fiber optic;wherein said using acontrollable material includes varying the index of refraction of thecontrollable material according to the sensed level of at least aportion of the optical energy transmitted in said fiber optic.
 16. Amethod for controllably attenuating optical energy transmitted through acore of a fiber optic, a portion of the fiber optic having a portion ofcladding encasing the core removed therefrom, thereby exposing anevanescent tail of the optical mode field transmitted through the core,the method comprising:extracting optical energy from the evanescent tailof the optical mode field using a bulk material, positioned over saidportion of the fiber optic, into which the evanescent tail penetrates;and using a controllable material positioned between the bulk materialand the core of the fiber optic to vary a depth of penetration of theevanescent tail into the bulk material including varying the index ofrefraction of the controllable material;wherein a majority of thecladding between an outer radius thereof and the core is removed fromthe portion of the fiber optic, and wherein: said using a controllablematerial includes varying an effective optical thickness thereof byvarying the index of refraction thereof, thereby varying the depth ofpenetration of the evanescent tail into the bulk material.
 17. A methodfor controllably attenuating optical energy transmitted through a coreof a fiber optic, a portion of the fiber optic having a portion ofcladding encasing the core removed therefrom, thereby exposing anevanescent tail of the optical mode field transmitted through the core,the method comprising:extracting optical energy from the evanescent tailof the optical mode field using a bulk material, positioned over saidportion of the fiber optic, into which the evanescent tail penetrates;and using a controllable material positioned between the bulk materialand the core of the fiber optic to vary a depth of penetration of theevanescent tail into the bulk material, including varying the index ofrefraction of the controllable material;wherein: the controllablematerial has a controllable index of refraction approximately matched tothe index of refraction of the cladding; and the bulk material has afixed index of refraction higher than the effective mode index ofrefraction of the fiber optic.
 18. A method for controllably attenuatingoptical energy transmitted through a core of a fiber optic, a portion ofthe fiber optic having a portion of cladding encasing the core removedtherefrom, thereby exposing an evanescent tail of the optical mode fieldtransmitted through the core, the method comprising:extracting opticalenergy from the evanescent tail of the optical mode field using a bulkmaterial, positioned over said portion of the fiber optic, into whichthe evanescent tail penetrates; and using a controllable materialpositioned between the bulk material and the core of the fiber optic tovary a depth of penetration of the evanescent tail into the bulkmaterial, including varying the index of refraction of the controllablematerial;wherein said using a controllable material includes: varyingthe index of refraction of the controllable material by changing atemperature or voltage stimulus applied thereto.
 19. A method forforming a controllable attenuator for attenuating optical energytransmitted through a fiber optic, comprising:removing material from aportion of the fiber optic thereby exposing a surface thereof throughwhich at least some of said optical energy transmitted therein can beextracted; forming a controllable material over the surface forcontrolling an amount of optical energy extracted from the fiber opticin accordance with refractive index changes induced therein by achangeable stimulus to be applied thereto; forming a bulk material overthe controllable material into which the extracted optical energy isradiated.
 20. A method for forming an attenuation system,comprising:forming a controllable attenuator in accordance with themethod of claim 19, and providing a control circuit coupled to thecontrollable attenuator for controlling a value of the changeablestimulus to be applied to the controllable attenuator.
 21. The method ofclaim 20, further comprising:providing a level sensing circuit coupledto the fiber optic for sensing the level of at least a portion of theoptical energy transmitted therein and providing a sensed level stimulusto said control circuit according to said sensed level;wherein thecontrol circuit is formed to control the value of the changeablestimulus to be applied to the controllable material in accordance withthe sensed level stimulus.
 22. The method of claim 19, wherein thematerial removed from the fiber optic is a portion of cladding encasinga core thereof, and wherein:the controllable material is formed to havea controllable index of refraction approximately matching the index ofrefraction of the cladding; and the bulk material formed over thecontrollable material is formed to have a fixed index of refractionhigher than the effective mode index of refraction of the fiber optic.23. The method of claim 19, wherein the changeable stimulus to beapplied to the controllable material comprises temperature or voltage.24. A method for forming an attenuation system for attenuating opticalenergy transmitted through a fiber optic, comprising:removing materialfrom a portion of the fiber optic thereby exposing a surface thereofthrough which at least some of said optical energy transmitted thereincan be extracted; forming a controllable material over the surface forcontrolling an amount of optical energy extracted from the fiber opticin accordance with refractive index changes induced therein by achangeable stimulus to be applied thereto; providing a level sensingcircuit, coupled to said fiber optic, for sensing a level of at least aportion of the optical energy transmitted therein and providing a sensedlevel stimulus; and providing a control circuit for controlling a valueof the changeable stimulus to be applied to the controllable material inaccordance with the sensed level stimulus, received from the levelsensing circuit.
 25. The method of claim 24, wherein the changeablestimulus to be applied to the controllable material comprisestemperature or voltage, and wherein the control circuit is formed toprovide a control stimulus to the controllable material to change thetemperature or voltage thereof.
 26. The method of claim 25, furthercomprising:providing a controllable heating element, or electrodes,having an input for receiving the control stimulus from said controlcircuit, and arranged with respect to the controllable material tochange the temperature, or voltage, and therefore the refractive indexthereof in accordance with said control stimulus.
 27. The method ofclaim 24, further comprising:forming a bulk material over thecontrollable material into which the extracted optical energy isradiated.
 28. The method of claim 27, wherein the material removed fromthe fiber optic is a portion of cladding encasing a core thereof, andwherein:the controllable material is formed to have a controllable indexof refraction approximately matching the index of refraction of thecladding; and the bulk material formed over the controllable material isformed to have a fixed index of refraction higher than the effectivemode index of refraction of the fiber optic.